FIG. 1 shows a front elevation view of a typical, prior-art front drive geometry for a two-wheel-drive (2WD) sport all-terrain vehicle (ATV), generally designated by reference numeral 10. The 2WD sport ATV has a frame 12 and a front suspension system connected to the frame for supporting each front wheel 14 having a diameter for receiving a tire of less than 11 inches in diameter. For the sake of simplicity, FIG. 1 only shows the left front wheel and the associated components of a left front drive assembly 20. The front drive assembly includes a front suspension assembly 30. The front suspension assembly includes an A-arm suspension system featuring an upper A-arm 32 and a lower A-arm 34, both of which are pivotally connected to the frame. FIG. 1 also shows a shock absorber 37 which is pivotally connected to the frame and to the lower A-arm. The upper and lower A-arms are pivotally connected to a knuckle 40 at upper and lower pivotal joints 36, 38 (which are preferably rod ends having spherical joints). The knuckle receives and supports a cast and press-fitted steel spindle 42. As is known in the art, a wheel hub 50 includes one or more bearings to facilitate rotation of hub relative to the spindle 42. As is known in the art, the wheel hub usually has a plurality of fasteners (typically four or five lugs) for fastening the wheel to the hub. A low-pressure balloon tire is installed on the rim of the wheel and inflated to a pressure usually no more than 2 kg/cm2 (which is 196 kPa or 28 psi). A disk brake 60 is affixed to an inner portion of the hub so that the tire, wheel, hub and disk brake rotate in unison with respect to the spindle, knuckle, and A-arms.
The upper and lower pivotal joints together define an axis of rotation known in the art as a “king pin axis” 80. When a driver turns the handlebars, the front wheel will rotate about the king pin axis. As shown in FIG. 1, the front wheel and low-pressure balloon tire defines a central plane of the wheel which is represented two-dimensionally by a wheel center line 70. When the wheel center line is vertical, the camber is said to be 0 degrees although small variations in the camber are common. Moreover, the camber seldom remains constant due to the compression of the suspension during ride. Due to the travel of the suspension, the camber will typically take on negative values, for instance less than −5 degrees. As is known in the art, the range of camber values depends on factors such as the turning characteristics of the vehicle and wheel travel constraints.
An important parameter governing vehicle dynamics and driver comfort is what is known in the art as a “scrub radius” 90. The scrub radius 90 is defined as the distance between the extended centerline of the steering axis (king pin axis) and the centerline of the tire where the tread contacts the ground. If the steering centerline is inboard of the tire centerline, the scrub radius is positive. If the steering centerline is outboard of the tire centerline, the scrub radius is negative. In other words, the scrub radius is defined as the distance between a first point where a downward projection of the king pin axis intersects the ground and a second point where a downward projection of the wheel center line intersects the ground. By convention, the scrub radius is measured using a frame of reference located at the second point and directed inwardly toward a central longitudinal axis of the vehicle. Alternatively, the scrub radius (SR) can be conveniently computed as SR=D1−D2 where D1 is the distance from the central longitudinal axis of the vehicle to the point where a downward projection of the king pin axis intersects the ground and D2 is the distance from the central longitudinal axis to the point where a downward projection of the wheel center line intersects the ground.
In all prior-art sport-type two-wheel-drive ATVs known to the Applicant, the scrub radius is positive because the 10-inch rims (which are customarily used on these sport vehicles to reduce the gyroscopic effect at higher speeds) are too small to permit tight packaging of the front wheel assemblies. In other words, in prior-art 2WD sport ATVs, the downward projection of the wheel center line intersects the ground at a point that is further outward from the central longitudinal axis of the vehicle than the point where the downward projection of the king pin axis intersects the ground. Stated otherwise, the king pin axis does not intersect the wheel center line above the ground.
Referring now to FIG. 2, the prior-art 2WD sport ATV having a positive scrub radius 90 has a tendency to suffer from steering kickback and lateral pull during braking. Lateral pull and steering kickback arise due to a moment M created when forces (FIMPACT and/or FGROUND) act about the scrub radius 90 (which acts as a lever arm for these forces). These forces typically result from braking or impacts on the front wheel. Impacts due to collision with obstacles (e.g. rocks, stumps or branches) can be at ground level or above ground.
At ground level, hard braking exerts a force FGROUND on the front wheel, especially when the ground adherence changes suddenly. Likewise, a ground-level obstacle will exert a force on the front wheel. This ground-level force FGROUND creates a turning moment M about the scrub radius 90 (which acts as a lever arm). These impacts cause the front wheel to “open”, i.e., the front wheel rotates undesirably in the direction of the impact. In turn, this causes the ATV to “pull” laterally. In order to compensate for this deleterious consequence of a positive scrub radius, the driver will often momentarily release the brakes, which will undesirably increase braking distance with the consequent effect of eroding the driver's confidence in the vehicle. In other words, overall braking performance is diminished.
Impacts that occur above ground are typically due to larger obstacles (e.g., rocks, stumps, logs or the like). Where the ATV is able to traverse the obstacle, the net force vector of the impact is usually lower than the axle of the front wheel. However, the moment M caused by this above-ground impact FIMPACT is proportional to the height of the impact above the ground. The effective lever arm is a function of both the scrub radius 90 and the length 44 of the spindle 42. The spindle length is, in turn, a function of the scrub radius and the angle of the king pin axis. The above-ground impact also tends to cause the front wheel to “open”, i.e. to rotate into the impact, which in turn means that the vehicle “pulls” laterally to the side where the impact occurred. A further consequence of this impact is steering kickback, which is felt by the driver as an unpleasant jolt in the handlebars as the handlebars are forcibly turned into the direction of the impact. Besides causing the vehicle to steer in a generally unwanted direction, the steering kickback is unpleasant and sometimes even painful for the driver. The magnitude of the steering kickback is an important consideration in the design of an ATV as it directly affects the driver's confidence in the vehicle's performance, particularly the driver's perception of the vehicle's capacity to rapidly, safely and comfortably traverse rocky or rugged terrain.
Efforts to reduce the scrub radius in ATVs (and hence to reduce the magnitude of these impact-induced moments) have been constrained primarily by space (“packaging”) requirements of the front drive assemblies and associated front suspension components. A typical prior-art front wheel assembly is shown in cross-section in FIG. 3. The front wheel assembly has a conventional, inwardly mounted disk brake 60, a conventional hub 50 with bearings 52, and a two-part, press-fitted knuckle 40 and spindle 42 (usually made of steel). The configuration of these components is illustrated in FIG. 3 and will not be described in greater detail as this type of front drive assembly is already well known to those of ordinary skill in the art. In the case of a sport-type two-wheel-drive ATV, the industry-standard 10-inch rims cannot accommodate these components using this traditional configuration without resulting in a positive scrub radius or a negative scrub radius combined with an undesirably large king pin axis angle. Because the configuration of the prior-art front drive assembly inevitably results in a geometry having a positive scrub radius, the prior-art ATVs suffer from lateral “pull” during braking as well as steering kickback.
Accordingly, there remains a need for a two-wheel-drive sport ATV which overcomes at least some of the deficiencies associated with front drive geometries having a positive scrub radius.